WLAN systems based on Institute of Electrical and Electronics Engineers, IEEE 802.11 standard utilises Industry, Scientific and Medical, ISM, radio bands. The medium access control of IEEE 802.11 is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The basic idea in CSMA/CA is that a wireless device (also referred to as STA) first senses the channel and if it is idle accesses it for transmission.
WLANs may use one of two modes; ad hoc mode or infrastructure mode. In ad hoc mode, a wireless device communicates with only one or a few other devices directly while in infrastructure mode, each device directly communicates with an Access Point, AP, to reach each another, or connect to the Internet. In this disclosure it is assumed that the infrastructure mode is used in a sensor monitoring scenario, where the data is delay tolerant. By delay tolerant is meant data that does not lose value if the delivery is delayed in the communication network. Also, it is assumed that a sensor WLAN STA, i.e. a wireless device in a WLAN acting as a sensor, is willing to save power when possible.
In the decentralised IEEE 802.11 MAC access principle based on CSMA/CA and implemented via the Distributed Coordination Function, DCF, or the Enhanced Distributed Channel Access, EDCA, different timing intervals are specified. The timing intervals of interest in this disclosure are for example: slot time, short inter-frame space, SIFS, distributed inter-frame space, DIFS, arbitration inter frame space, AIFS, and extended inter-frame space, EIFS.
When a wireless device wants to transmit, the medium is checked for occupancy; this procedure is called Clear Channel Assessment, CCA. If the medium is idle for a predefined period (e.g. DIFS or EIFS) plus a back-off time defined as a random number of slots chosen from a Contention Window, CW, the transmission will start as establish in 802.11 systems. If the medium is considered busy, the wireless device will defer its transmission until the medium is idle. The size of the CW is doubled with each unsuccessful transmission.
The CCA operates through two main methods, signal detection CCA, and energy detection CCA.
The signal detect CCA consists of a physical signal detect and a virtual signal detect. The physical signal detect is carried out directly on the PHY layer and is a direct measurement of the received signal strength of a valid 802.11 transmission through preamble detection. The virtual signal detect is provided by the Media Access Control, MAC, layer and uses the Network Allocation Vector, NAV, functionality. In essence, the NAV is a virtual carrier-sensing mechanism that contains a timer, which defers the wireless device from accessing the medium a certain time. The value of the timer is decided by reading the duration length of a preceding packet and adding additional IFS time e.g. DIFS. By using the NAV, the wireless device does not need to constantly monitor the environment for example it may sleep during NAV and may thus save energy. In 802.11ah, there is also a secondary virtual carrier sensing mechanism called Response Indication Deferral, RID, with a similar functionality as the NAV. Both the NAV and the RID counters must have reached zero before the wireless device can access the medium.
Energy detect CCA integrates the signal strength from the radio frontend during the CCA window. Thus, no knowledge of the type of data is required. This energy level is then compared to the noise floor (derived from the background noise) to make a decision whether the medium is occupied or not. The main advantages of energy detect CCA, compared to signal detect CCA are: low complexity, generality, and low power consumption. It has low complexity since no decoding of the signal is required, it is general because no information about the received signal is necessary, and it does not require much power since no receiver complexity besides an energy detector is required. The CCA module can be a simple non-coherent module. Unlike the signal detect CCA; there is no need to wait for synchronisation and detection of the underlying signal. However, the energy detect CCA also suffers one significant drawback compared to the signal detect CCA; in order of accurately detect the presence of an interferer, the energy detect CCA requires a signal strength that is in the order of 20 dB stronger compared to the signal detect CCA.
The 802.11ah amendment introduced a new sensor wireless device, also referred to as a sensor STA, using data frames with small payload size and expected low duty cycle, and low traffic volumes. These sensor wireless devices are typically deployed in static locations. Furthermore, the sensor wireless devices may be deployed in big numbers in locations that are difficult to access; thus a long battery life (up to several years) is essential. Therefore, power efficiency and low complexity are key features in these wireless devices. The sort of measurement performed by the sensor wireless devices is typically known, and it makes sense that the data transmitted by sensor wireless devices is predictable (and often also small); furthermore it may often fit into one single transmission packet.
A Basic Service Set, BSS, is usually maintained by a single AP; it can suffer interference from other WLAN systems or interference from other radio systems which are not WLAN, operating the same frequency band. Examples of such systems may be Bluetooth, 802.15.4, or Long Term Evolution License Assisted Access, LTE LAA.
LTE LAA is a feature which allows LTE to take advantage of unlicensed spectrum which is normally used for example by WLAN systems. In general the principle is to allow a fair coexistence between LTE LAA and WLAN from the point of view of air-time.
A WLAN wireless device may potentially decode other WLAN transmissions not intended to itself, and thus, a WLAN can use signal detect CCA, decode the packet and set its NAV/RID timer to defer from access the channel. However, a WLAN wireless device cannot decode a non-WLAN technology and thus it keeps accessing the channel using energy detection until it is free in order to start transmission which consumes unnecessary power.
Since the medium (i.e. radio resources of one or more channels) is shared among many wireless devices, access to the communication medium follows the aforementioned CSMA/CA methodology. If there may be non-WLAN systems present in the same frequency band, they will cause considerable interference. Then, sensor wireless devices will waste power accessing the channel busy/idle because wireless devices keep accessing the channel using energy detection until it is free in order to start transmission s. Since long battery life is an essential feature in the sensor type wireless devices, this power waste due to channel sensing is undesirable.